Glass composition, glass article, glass substrate for magnetic recording media, and method for producing the same

ABSTRACT

A glass substrate of the present invention for magnetic recording media having high heat resistance and easy chemical strengthening ability at once has not been obtained, which is a glass composition essentially comprising 60 to 70 wt % SiO 2 , 5 to 20 wt % Al 2 O 3 , 0 to 1 wt % Li 2 O, 3 to 18 wt % Na 2 O, 0 to 9 wt % K 2 O, 0 to 10 wt % MgO, 1 to 15 wt % CaO, 0 to 4.5 wt % SrO, 0 to 1 wt % BaO, 0 to 1 wt % TiO 2  and 0 to 1 wt % ZrO 2 , wherein the sum of Li 2 O, Na 2 O and K 2 O is from 5 to 25 wt %, and the sum of MgO, CaO, SrO and BaO is from 5 to 20 wt %.

FIELD OF THE INVENTION

The present invention relates to a glass composition having high heatresistance and capable of giving a high degree of strengtheningaccording to a chemical strengthening treatment by an ion exchange, anda glass article containing the glass composition. In particular, theinvention relates to a glass substrate useful for magnetic recordingmedia. Further, the invention relates to a method for producing a sheetglass having high heat resistance and capable of giving a high degree ofstrengthening according to a chemical strengthening treatment by an ionexchange.

DESCRIPTION OF THE RELATED ART

Glass has excellent properties which the other substances do not have,such as high surface smoothness and high surface hardness, and issuitable for substrates for information recording media, such as harddisk drives (magnetic recording devices).

However, glass has the disadvantage that it is easily broken or cracked.As a countermeasure against this, a compression stress has been given toa surface of glass by rapid cooling or an ion exchange, that is to say,a so-called strengthening treatment has been conducted. Above all, achemical strengthening treatment by an ion exchange is suitable formaterials for substrates requiring particularly high dimensionalaccuracy, because of exceedingly small deformation of glass.

Japanese Patent No. 2,837,134 discloses a glass substrate forinformation recording formed from a chemically strengthened glassobtained by subjecting a glass for chemical strengthening containing 62to 75 wt % SiO₂, 5 to 15 wt % Al₂O₃, 4 to 10 wt % LiO₂, 4 to 12 wt %Na₂O and 5.5 to 15 wt % ZrO₂, and having an Na₂O/ZrO₂ ratio of 0.5 to2.0 by weight and an Al₂O₃/ZrO₂ ratio of 0.4 to 2.5 by weight to anion-exchange treatment with a molten salt containing Na ions and/or Kions. However, this glass has the disadvantage that the heat resistanceof the glass itself is low even when it is subjected to the chemicalstrengthening treatment by the ion exchange. That is to say, with arecent increase in recording density, the layer constitution of amagnetic material formed on a substrate has been complicated and highlydeveloped, and there has been a tendency to necessitate an increase insubstrate temperature in forming the layers. In particular, a magneticmaterial for perpendicular magnetic recording which is hereafterexpected to become the mainstream necessitates a particularly hightemperature (for example, 400° C. or higher) in forming it. At the hightemperature, the problem that a roughness in surface of the glass or adeformation of the glass is caused, has been encountered. For example,in a conventional production process of a glass for a magnetic discsubstrate, a polishing of the glass surface is carried out. A residualstress is caused in the glass surface, since pressure is put on theglass surface at the polishing. When the glass in state of the above isexposed to a high temperature, a thermal relaxation of the residualstress is caused, and projections are formed in the glass surface. As aresult, the glass surface become rough. The glass warps and deforms byheat at the high temperature.

JP-A-9-2836 (the term “A” as used herein means laid-open publication)discloses a glass substrate for magnetic disks obtained by subjecting aglass containing 50 to 65 wt % SiO₂, 5 to 15 wt % Al₂O₃, 2 to 7 wt %Na₂O, 4 to 9 wt % K₂O (the sum of Na₂O and K₂O is from 7 to 14 wt %), 12to 25 wt % (MgO+CaO+SrO+BaO) and 1 to 6 wt % ZrO₂ to a chemicalstrengthening treatment. However, although this glass substrate has highheat resistance, there are problems; the strength thereof isinsufficient to withstand the centrifugal force of a recent magneticdisk drive (HDD) in which a magnetic recording medium is driven forrotation at high speed, or the collision of a magnetic recording mediumwith a recording head in a so-called load-unload system in which themagnetic recording medium gets off from the recording head in stoppingthe magnetic recording medium and the recording head is loaded on themagnetic recording medium in rotating.

SUMMARY OF THE INVENTION

The invention has been made taking it as a technical subject to solvethe above-mentioned problems. An object of the invention is to provide aglass that a roughness in surface of the glass or a deformation of theglass is not caused, even when it is subjected to a high-temperaturetreatment in producing a magnetic recording medium, and further obtainshigh mechanical strength according to a chemical strengthening treatmentby an ion exchange. Another object of the invention is to provide amethod for producing the glass.

According to the invention, there is provided (1) a glass compositionessentially comprising:

60 to 70 wt % SiO₂;

5 to 20 wt % Al₂O₃;

0 to 1 wt % Li₂O;

3 to 18 wt % Na₂O;

0 to 9 wt % K₂O;

0 to 10 wt % MgO;

1 to 15 wt % CaO;

0 to 4.5 wt % SrO;

0 to 1 wt % BaO;

0 to 1 wt % TiO₂; and

0 to 1 wt % ZrO₂,

wherein the sum of Li₂O, Na₂O and K₂O is from 5 to 25 wt %, and the sumof MgO, CaO, SrO and BaO is from 5 to 20 wt %.

A chemical strengthening treatment can give high mechanical strength toa glass article obtained from the glass composition of (1). Even whenthe glass is subjected to a high-temperature heat treatment, the glassis not deformed by heat. Further, no unevenness caused by alkali elutionis formed on a surface thereof.

There is further provided (2) the glass composition according to (1),which essentially comprises:

60 to 70 wt % SiO₂;

8 to 15 wt % Al₂O₃;

8 to 16 wt % Na₂O;

0 to 3.5 wt % K₂O;

2 to 5 wt % MgO;

3 to 7.5 wt % CaO;

0 to 4.5 wt % SrO; and

0 to 1 wt % ZrO₂,

wherein the sum of Na₂O and K₂O is from 11 to 18 wt %, and the sum ofMgO, CaO and SrO is from 7 to 14 wt %.

More certainly, a chemical strengthening treatment can give highermechanical strength to a glass article obtained from the glasscomposition of (2). Even when the glass is subjected to ahigh-temperature heat treatment, the glass is not deformed morecertainly. Further, unevenness caused by alkali elution is morecertainly prevented from being formed on a surface thereof.

The invention provides (3) the glass composition of (1) or (2), whichhas a glass transition point of 560° C. or higher.

Further, the invention provides (4) the glass composition of any one of(1) to (3), which has a thermal expansion coefficient of 70×10⁻⁷/° C. orhigher as measured in the range of from −50° C. to 70° C., and a thermalexpansion coefficient of 80×10⁻⁷/° C. or higher as measured in the rangeof from 50° C. to 350° C.

According to the glass composition of (4), when a glass having the glasscomposition is adhered or joined to a metal, occurrence of the strain,displacement or crack breakage of the glass caused by the difference inthe coefficient of thermal expansion is inhibited, because thecoefficient of thermal expansion of the glass approximates to that ofthe metal material.

According to the invention, there is further provided (5) a chemicallystrengthened glass article obtained by immersing a glass articlecomprising the glass composition described in any one of (1) to (4) in amolten salt containing monovalent cations having an ionic radius largerthan that of Na ions to conduct an ion-exchange treatment between the Naions and the monovalent cations.

According to the glass article of (5), a compression stress layer isformed on a surface of the glass article, so that the glass article canbe prevented from being broken even when shock is given from theoutside.

The invention provides (6) a glass substrate for magnetic recordingmedia, essentially comprising:

60 to 70 wt % SiO₂;

5 to 20 wt % Al₂O₃;

0 to 1 wt % Li₂O;

3 to 18 wt % Na₂O;

0 to 9 wt % K₂O;

0 to 10 wt % MgO;

1 to 15 wt % CaO;

0 to 4.5 wt % SrO;

0 to 1 wt % BaO;

0 to 1 wt % TiO₂; and

0 to 1 wt % ZrO₂,

wherein the sum of Li₂O, Na₂O and K₂O is from 5 to 25 wt %, and the sumof MgO, CaO, SrO and BaO is from 5 to 20 wt %.

According to the glass substrate of (6), even when the glass substrateis subjected to high-temperature heating in forming a magnetic recordinglayer on a surface thereof, a roughness in surface of the glass or adeformation of the glass is caused, and concavo-convex formationattributable to an alkali elution are not caused on the surface of thesubstrate to allow the surface to be kept smooth.

The invention further provides (7) the glass substrate for magneticrecording media according to (6), which essentially comprises:

60 to 70 wt % SiO₂;

8 to 15 wt % Al₂O₃;

8 to 16 wt % Na₂O;

0 to 3.5 wt % K₂O;

2 to 5 wt % MgO;

3 to 7.5 wt % CaO;

0 to 4.5 wt % SrO; and

0 to 1 wt % ZrO₂,

wherein the sum of Na₂O and K₂O is from 11 to 18 wt %, and the sum ofMgO, CaO and SrO is from 7 to 14 wt %.

According to the glass substrate of (7), even when the glass substrateis subjected to high-temperature heating in forming a magnetic recordinglayer on a surface thereof, a roughness in surface of the glass or adeformation of the glass is caused with even more efficient, andconcavo-convex formation attributable to an alkali elution are notcaused on the surface of the substrate with even more efficient, toallow the surface to be kept more smooth.

The invention provides (8) the glass substrate for magnetic recordingmedia described in (6) or (7), which has a glass transition point of560° C. or higher.

According to the glass substrate of (8), even when the glass substrateis subjected to heating in a molten salt in chemically strengthening theglass substrate, or heating in forming a magnetic recording layer on theglass substrate, occurrence of warping in the glass substrate can beprevented. Further, in forming the magnetic recording layer, formationof projections attributable to an alkali elution on a surface of theglass substrate can be inhibited.

There is further provided (9) the glass substrate for magnetic recordingmedia described in any one of (6) to (8), which has a thermal expansioncoefficient of 70×10⁻⁷/° C. or higher as measured in the range of from−50° C. to 70° C., and a thermal expansion coefficient of 80×10⁻⁷/° C.or higher as measured in the range of from 50° C. to 350° C.

According to the glass substrate of (9), the coefficient of thermalexpansion of the glass substrate approximates to that of the metal,particularly stainless steel. Accordingly, even when the glass substrateis attached to a metal rotational axis of a magnetic recording deviceand driven for rotation at high speed, occurrence of the followingdefects can be prevented: changes in dimension and warping of the glassby heat is not generated and thereby the glass does not deviate from arotational axis during a rotation, in addition the position of the headdoes not deviate during a driving of disc.

The invention provides (10) a chemically strengthened glass substratefor magnetic recording media obtained by immersing the glass substrateof any one of (6) to (9) in a molten salt containing monovalent cationshaving an ionic radius larger than that of Na ions to conduct anion-exchange treatment between the Na ions and the monovalent cations.

According to the chemically strengthened glass substrate of (10), acompression stress layer is formed deeply in a surface of the glasssubstrate, so that breakage of the substrate by external force can beprevented.

The invention further provides (11) the chemically strengthened glasssubstrate for magnetic recording media described in (10), wherein theload at which a crack is developed with a probability of 50% bydepressing a diamond penetrator of a micro Vickers hardness testertoward the substrate is 800 g or more.

According to the chemically strengthened glass substrate of (11), theload at which a crack is developed with a probability of 50% bydepressing a diamond penetrator of a micro Vickers hardness testertoward the substrate is 800 g or more. Accordingly, even when externalforce is applied to a surface of the glass substrate to form animpression thereon, the probability of vertically developing a crackaround the impression can be reduced. This can inhibit a reduction inglass strength, even when in a process of producing a magnetic recordingmedium by forming a magnetic recording layer, the glass substrate isplaced on stainless steal support fittings and moved, or transferredbetween jigs to form fine scratches on a surface of the glass substrate.In a device in which a recording head gets off out of a disk instopping, that is, a device having a ramped loading which has nowprevailed among magnetic recording devices, the substrate can beprevented from being cracked by shock generated when the recording headis loaded on the medium in starting.

The invention provides (12) a method for producing a sheet glass by afloat process comprising blending raw materials for a glass so as togive a molten glass having the following composition, and introducingthe molten glass obtained by melting the raw materials onto a bath oftin to form it in a sheet shape:

60 to 70 wt % SiO₂;

5 to 20 wt % Al₂O₃;

0 to 1 wt % Li₂O;

3 to 18 wt % Na₂O;

0 to 9 wt % K₂O;

0 to 10 wt % MgO;

1 to 15 wt % CaO;

0 to 4.5 wt % SrO;

0 to 1 wt % BaO;

0 to 1 wt % TiO₂; and

0 to 1 wt % ZrO₂,

wherein the sum of Li₂O, Na₂O and K₂O is from 5 to 25 wt %, and the sumof MgO, CaO, SrO and BaO is from 5 to 20 wt %.

According to the method of (12), the molten glass can be directlyintroduced from a glass-melting furnace onto a molten tin bath, andformed into a sheet shape. This makes it possible to obtain raw sheetglasses for obtaining the glass substrates for magnetic recording mediahaving a specified thickness in large amounts.

There is further provided (13) the method for producing a sheet glass bya float process according to (12), which comprises blending rawmaterials for a glass so as to give a molten glass having the followingcomposition, and introducing the molten glass obtained by melting theraw materials onto a bath of tin to form it in a sheet shape:

60 to 70 wt % SiO₂;

8 to 15 wt % Al₂O₃;

8 to 16 wt % Na₂O;

0 to 3.5 wt % K₂O;

2 to 5 wt % MgO;

3 to 7.5 wt % CaO;

0 to 4.5 wt % SrO; and

0 to 1 wt % ZrO₂,

wherein the sum of Na₂O and K₂O is from 11 to 18 wt %, and the sum ofMgO, CaO and SrO is from 7 to 14 wt %.

According to the method of (13), the molten glass can be directlyintroduced from a glass-melting furnace onto a molten tin bath, andformed into the sheet shape more effectively without formation of anunmelted or devitrified material of the glass. This makes it possible toobtain raw sheet glasses for obtaining glass substrates for magneticrecording media having a specified thickness in large quantities.

The invention provides (14) the method of (12) or (13), wherein thesheet glass has a glass transition point of 560° C. or higher.

According to the method of (14), the glass transition point is 560° C.or higher, so that heat resistance at high temperatures can be ensured.

The invention further provides (15) the method of any one of (12) to(14), wherein the sheet glass has a thermal expansion coefficient of70×10⁻⁷/° C. or higher as measured in the range of from −50° C. to 70°C., and a thermal expansion coefficient of 80×10⁻⁷/° C. or higher asmeasured in the range of from 50° C. to 350° C.

According to the method of (15), raw sheet glasses suitable for theglass substrates for magnetic recording media can be formed.

DETAILED DESCRIPTION OF THE INVENTION

Modes for carrying out the invention will be explained below in detail.Reasons for limitations on the content of components of the glasscomposition of the invention will be explained below. Percentages are byweight in the following descriptions.

SiO₂ is a main component constituting the glass. When the contentthereof is lower than 60%, the glass is deteriorated in heat resistanceand chemical durability. On the other hand, exceeding 70% results in anincrease in viscosity of the glass at high temperatures, which causesdifficulty in melting and forming it. The content of SiO₂ is thereforerequired to be from 60 to 70%.

Al₂O₃ is an indispensable component for improving the heat resistanceand chemical durability of the glass and facilitating chemicalstrengthening. When the content thereof is lower than 5%, these effectsare not sufficiently achieved. On the other hand, when the contentthereof exceeds 20%, the liquidus temperature of the glass is elevatedto deteriorate formability thereof into a sheet shape. The content ofAl₂O₃ is therefore required to be from 5 to 20%. It is preferably 8% orhigher and 15% or lower for attaining a proper balance between the heatresistance (glass transition point) and chemical durability of theglass.

Li₂O, Na₂O and K₂O (these are hereinafter generically named R₂O) arecomponents for reducing the viscosity of the glass to facilitate meltingof the glass, lowering the liquidus temperature to enhance theformability into a sheet shape, and further heightening the coefficientof thermal expansion. In order to obtain these effects, the sum of thethree components is required to be 5% or higher. On the other hand, whenthe sum of these components exceeds 25%, the heat resistance of theglass deteriorates, and further, the chemical durability deteriorates.Accordingly, the total content of Li₂O, Na₂O and K₂O is required to befrom 5 to 25%. Furthermore, the total content of these is preferably 11%or higher and 18% or lower for attaining a proper balance between theheat resistance and chemical durability of the glass.

Of R₂O, Li₂O is particularly a component for improving the strength ofthe glass by substitution of Li ions with other cations such as Na ionsand K ions in the molten salt. However, Li₂O has the disadvantage thatan increase in its content impairs the heat resistance of the glass. Thecontent of Li₂O is therefore required to be 1% or lower, and it ispreferred that the content of Li₂O is substantially an impurity amount.

Na₂O is a component for improving the strength of the glass bysubstitution with other cations such as K ions in the molten salt. OfR₂O, raw materials therefor are inexpensive and easily available. Inrespect to raw material cost, it is preferred that the ratio of Na₂O toR₂O is increased. When the content is lower than 3%, it becomesdifficult to obtain the chemically strengthened glass having sufficientstrength, resulting in insufficient manifestation of the effect. Fromthis viewpoint, it is preferred that the content is 8% or higher. On theother hand, when the content exceeds 18%, the heat resistance of theglass is largely impaired, so that the content is required to be 18% orlower. For ensuring the heat resistance of the glass more certainly, thecontent is preferably 15% or lower.

Of R₂O, K₂O has the advantage that the heat resistance is hard to beimpaired. However, when the content thereof exceeds 9%, a compressionstress necessary for ensuring strength by an ion-exchange treatment witha molten salt of potassium nitrate ordinarily used can not be formed ina surface of the glass. The content of K₂O is therefore required to befrom 0 to 9%, and is preferably from 0 to 3.5%.

MgO, CaO, SrO and BaO (these are hereinafter generically named RO) arecomponents for reducing the viscosity of the glass to facilitate meltingof the glass. Further, these have the effect of heightening thecoefficient of thermal expansion, although a contribution thereto issmall compared to R₂O . In order to obtain these effects, the sum of thefour components is required to be 5% or higher, and is preferably 7% orhigher. On the other hand, when the sum of these components exceeds 20%,it becomes difficult to chemically strengthen the glass to obtain thestrengthened glass. The sum is therefore required to be 20% or lower.Exceeding 14% results in a rise in the devitrification temperature ofthe glass, which is unfavorable in that it becomes difficult to form thesheet glass by the float process in which the molten glass is directlyintroduced from a glass-melting furnace onto a molten tin bath andformed into a sheet shape. Accordingly, the total amount of RO ispreferably 7% or higher and 14% or lower for obtaining the chemicallystrengthened glass and making it possible to directly forming the sheetglass by the float process.

Of RO, MgO has the advantage that it is hard to exert an adverse effecton chemical strengthening. However, MgO has a strong tendency to elevatethe devitrification temperature of the glass, so that the content of MgOis required to be from 0 to 10%. From the viewpoint of maintenance ofchemical properties of the glass, the content of MgO is preferably 2% orhigher, and from the viewpoint of inhibition of a devitrificationphenomenon of the glass, it is preferably 5% or lower.

Of RO, CaO does not exert a significant adverse effect on thedevitrification temperature of the glass, and is an indispensablecomponent for inhibiting an adverse effect on chemical strengthening andimproving meltability, compared to SrO. Less than 1% results ininsufficient manifestation of the effect, whereas exceeding 15% resultsin a rise in the devitrification temperature of the glass to deteriorateglass formability. The content of CaO is therefore required to be from 1to 15%. From the viewpoints of preparation of the glass which can bechemically strengthened and ensuring of meltability of the glass, thecontent of CaO is preferably 3% or higher. Further, from the viewpointof inhibition of a rise in the devitrification temperature of the glass,it is preferably 7.5% or lower.

Of RO, SrO particularly has the advantage that it does not elevate thedevitrification temperature. However, SrO has the property of preventingthe transfer of an alkali (R₂O) contained in the glass. Accordingly,when the content of SrO exceeds 4.5%, chemical strengthening becomesdifficult. Further, a large quantity of SrO contained in the glassincreases the density of the glass. The content of SrO is thereforerequired to be 4.5% or lower.

Of RO, BaO particularly has the advantage that it dose not elevate thedevitrification temperature. However, of RO, BaO has the property ofmost preventing the transfer of an alkali contained in the glass.Accordingly, when the content of BaO increases, it becomes difficult tochemically strengthen the glass by an ion exchange. Further, a largequantity of BaO contained in the glass increases the density of theglass. Furthermore, a barium raw material is a poisonous substance, sothat handling thereof is difficult. The content of BaO is thereforerequired to be 1% or lower, and it is preferred that the content of BaOis substantially an impurity amount.

TiO₂ is a component for improving meltability without deteriorating theheat resistance of the glass. When the content of TiO₂ exceeds 1%, thedevitrification temperature of the glass is elevated to deteriorateformability thereof. Further, TiO₂ colors the glass yellow by thecoexistence with iron contained in the glass, which is caused by irongenerally contained as an impurity in raw materials for glass.Accordingly, a difficulty is encountered with regard to recycling ofTiO₂-containing glass. The content of TiO₂ is therefore required to be1% or lower, and it is preferred that the content of TiO₂ issubstantially an impurity amount.

ZrO₂ is a component for improving the heat resistance of the glass. Whenthe content of ZrO₂ exceeds 1%, the devitrification temperature of theglass is elevated to deteriorate formability there of into a sheetshape. The content of ZrO₂ is therefore required to be 1% or lower.

In the invention, components other than the above-mentioned components,for example, Sb₂O₃, As₂O₅, SO₃, SnO₂ and F in a fluorine-containingcompounds as glass clarifying agents for defoaming in melting,transition metal compounds such as Fe₂O₃, CoO and NiO for coloring theglass, and other impurities of glass raw material origin, can each becontained within the range not exceeding 0.5% by weight.

When the glass components are selected within the above-mentionedcontent ranges in the glass composition of the invention, and theworking temperature of the glass and the devitrification temperature ofthe glass indicated by ° C. satisfy the relationship (a value of theworking temperature)−(a value of the devitrification temperature)≦−17,the glass preferable for directly forming the sheet glass by the floatprocess in which the molten glass is directly introduced from aglass-melting furnace onto a molten tin bath and formed into a sheetshape can be obtained.

The glass composition of the invention has a glass transition point of560° C. or higher. Accordingly, the properties thereof are notdeteriorated, for example, even when it is subjected to a heatingprocess in forming the magnetic recording layer on the glass substrateby sputtering film formation. In particular, the composition is suitablefor the substrate for perpendicular magnetic recording media, which isheated at high temperatures.

The glass composition of the invention has a thermal expansioncoefficient of 7×10³¹ ⁷/° C. or higher as measured in the range of from−50° C. to 70° C., and a thermal expansion coefficient of 80×10⁻⁷/° C.or higher as measured in the range of from 50° C. to 350° C.Accordingly, even when the glass composition is adhered or joined to ametal material, no crack caused by the difference in expansion betweenthe materials by changes in temperature is developed in the glass, andfurthermore, no breakage thereof occurs. In respect to a phenomenon ofdimensional expansion and contraction of the glass by changes intemperature, for example, even when a recording track of the magneticrecording medium is narrowed, a tracking error caused by the differencein thermal expansion between the glass and the metal structural materialcan be inhibited or avoided.

The glass composition of the invention can be enhanced in strength bybringing it in contact with a molten salt containing monovalent cationshaving an ionic radius larger than that of Na ions, for example,potassium nitrate or a mixed salt of potassium nitrate and sodiumnitrate, to give a surface compression stress by an ion exchange. Thecomposition is therefore suitable for a high-speed rotary magnetic diskdrive (HDD). The glass substrate obtained from the glass composition ofthe invention can ensure sufficient strength even when thinned.Accordingly, the substrate can be used as a substrate for a panel suchas a liquid crystal display or the like.

EXAMPLES

The invention will be explained below in more detail by reference toexamples. Glasses of Examples 1 to 19 having glass compositions of theinvention were prepared by melting experiments, and the meltingtemperature, the working temperature, the glass transition point, thecoefficient of linear expansion, the specific gravity, the Young'smodulus and the pressure at which a crack is developed with aprobability of 50% were measured for the resulting glasses. Resultsthereof are shown in Tables 1 and 2. Further, a glass disclosed inExample 3 of Japanese Patent No. 2,837,134 and a glass disclosed inJP-A-9-2836 were prepared by melting experiments, and results thereofare shown as Comparative Examples 1 and 2, respectively, in Table 1.

The preparation of the glassed of Examples 1 to 19 and ComparativeExamples 1 and 2 and measurements of the properties of the resultingglasses were conducted according to the following procedures.

TABLE 1 Example Example Example Example Item 1 2 3 4 Composition SiO₂61.7 60.8 62.3 62.2 (% by weight) Al₂O₃ 10.5 10.3 10.5 14.1 Li₂O 0.0 0.00.0 0.0 Na₂O 9.8 9.6 9.8 14.1 K₂O 6.4 5.8 6.0 0.0 MgO 3.2 1.9 3.2 3.3CaO 7.7 7.6 8.2 6.1 SrO 0.8 4.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.00.0 0.0 ZrO₂ 0.0 0.0 0.0 0.3 R₂O 16.2 15.4 15.8 14.1 RO 11.7 13.5 11.49.4 Melting temperature (log 1522 1540 1545 1535 η = 2) [° C.] Workingtemperature 1089 1094 1097 1103 (log η = 4) [° C.] Devitrification 11581147 1156 1140 temperature [° C.] Working temperature −69 −53 −59 −37 [°C.] − Devitrification temperature [° C.] Glass Transition Point 584 583588 606 [° C.] Coefficient of linear 94 96 94 86 expansion α (50 to 350°C.) [×10⁻⁷/° C.] Coefficient of linear 79 82 80 73 expansion α (−50 to70° C.) [×10⁻⁷/° C.] Specific gravity [g/cm³] 2.2 2.6 2.5 2.5 Young'smodulus [GPa] 75 77 76 76 Load at which a crack is 800 800 1400 >2000developed with a probability of 50% [g] Example Example Example ExampleItem 5 6 7 8 Composition SiO₂ 63.5 65.3 65.3 64.3 (% by weight) Al₂O₃10.4 10.6 10.6 10.5 Li₂O 0.0 0.0 0.0 0.0 Na₂O 9.7 13.9 13.9 9.8 K₂O 1.90.0 0.0 2.2 MgO 2.0 3.2 3.2 3.2 CaO 7.5 6.0 6.0 5.9 SrO 4.1 0.0 0.0 4.1BaO 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 1.0 0.0 ZrO₂ 1.0 1.0 0.0 0.0 R₂O 11.613.9 13.9 12.0 RO 13.6 9.2 9.2 13.2 Melting temperature (log 1562 15501547 1597 η = 2) [° C.] Working temperature 1120 1093 1091 1131 log η =4) [° C.] Devitrification 1155 1126 1123 1119 temperature [° C.] Workingtemperature −35 −33 −32 12 [° C.] − Devitrification temperature [° C.]Glass Transition Point 611 594 590 605 [° C.] Coefficient of linear 8285 85 79 expansion α (50 to 350° C.) [×10⁻⁷/° C.] Coefficient of linear72 73 73 71 expansion α (−50 to 70° C.) [×10⁻⁷/° C.] Specific gravity[g/cm³] 2.6 2.5 2.5 2.5 Young's modulus [GPa] 77 76 76 77 Load at whicha crack is 900 >2000 >2000 1200 developed with a probability of 50% [g]Compara- Compara- Example Example tive tive Item 9 10 Example 1 Example2 Composition SiO₂ 64.6 61.1 58.0 63.0 (% by weight) Al₂O₃ 10.5 10.3 7.014.0 Li₂O 0.0 0.0 0.0 6.0 Na₂O 9.8 9.7 4.2 10.0 K₂O 2.2 5.9 6.3 0.0 MgO4.5 3.1 2.0 0.0 CaO 4.2 5.9 4.8 0.0 SrO 4.1 4.0 6.9 0.0 BaO 0.0 0.0 7.90.0 TiO₂ 0.0 0.0 0.0 0.0 ZrO₂ 0.0 0.0 2.9 7.0 R₂O 12.1 15.5 10.5 16.0 RO12.8 13.0 21.6 0.0 Melting temperature 1612 1535 1545 1491 (log η = 2)[° C.] Working temperature 1150 1092 1145 1047 (log η = 4) [°C.]Devitrification 1137 1081 1070 900 temperature [° C.] Workingtemperature 13 11 75 147 [° C.] − Devitrification temperature [° C.]Glass Transition Point 602 579 615 491 [° C.] Coefficient of linear 8295 84 93 expansion α (50 to 350° C.) [×10⁻⁷/° C.] Coefficient of linear72 80 72 74 expansion α (−50 to 70° C.) [×10⁻⁷/° C.] Specific gravity2.5 2.6 2.8 2.5 [g/cm³] Young's modulus [GPa] 77 75 76 83 Load at whicha crack is 1800 1200 100 >2000 developed with a probability of 50% [g]

TABLE 2 Example Example Example Example Item 11 12 13 14 CompositionSiO₂ 61.4 63.3 65.9 66.9 (% by weight) Al₂O₃ 10.4 10.5 10.7 10.7 Li₂O0.0 0.0 0.0 0.0 Na₂O 9.7 12.9 14.0 13.0 K₂O 5.9 0.0 0.0 0.0 MgO 4.4 3.23.3 3.3 CaO 4.1 6.0 6.1 6.1 SrO 4.1 4.1 0.0 0.0 BaO 0.0 0.0 0.0 0.0 TiO₂0.0 0.0 0.0 0.0 ZrO₂ 0.0 0.0 0.0 0.0 R₂O 15.6 12.9 14.0 13.0 RO 12.613.3 9.3 9.3 Melting temperature (log 1538 1536 1531 1606 η = 2) [° C.]Working temperature 1093 1091 1101 1133 (log η = 4) [° C.]Devitrification 1087 1093 1106 1150 temperature [° C.] Workingtemperature 6 −2 −5 −17 [° C.] − Devitrification temperature [° C.]Glass Transition Point 582 591 590 598 [° C.] Coefficient of linear 8986 89 80 expansion α (50 to 350° C.) [×10⁻⁷/° C.] Coefficient of linear75 73 75 71 expansion α (−50 to 70° C.) [×10⁻⁷/° C.] Specific gravity[g/cm³] 2.5 2.6 2.5 2.5 Young's modulus [GPa] 76 77 75 74 Load at whicha crack is 900 1400 >2000 >2000 developed with a probability of 50% [g]Example Example Example Example Item 15 16 17 18 Composition SiO₂ 65.665.1 66.4 66.6 (% by weight) Al₂O₃ 10.7 10.6 10.8 9.9 Li₂O 0.0 0.0 1.00.0 Na₂O 14.0 13.9 12.1 14.0 K₂O 0.0 0.0 0.0 0.0 MgO 2.9 2.9 3.3 3.2 CaO6.5 5.6 6.1 6.1 SrO 0.0 1.7 0.0 0.0 BaO 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.00.0 ZrO₂ 0.3 0.3 0.3 0.3 R₂O 14.0 13.9 13.1 14.0 RO 9.4 10.1 9.4 9.3Melting temperature 1567 1563 1549 1565 (log η = 2) [° C.] Workingtemperature 1099 1096 1089 1097 (log η = 4) [° C.] Devitrification 11061079 1120 1115 temperature [° C.] Working temperature −7 17 −31 −18 [°C.] − Devitrification temperature [° C.] Glass Transition Point 591 586560 587 [° C.] Coefficient of linear 86 87 84 82 expansion α (50 to 350°C.) [×10⁻⁷/° C.] Coefficient of linear 74 74 73 72 expansion α (−50 to70° C.) [×10⁻⁷/° C.] Specific gravity [g/cm³] 2.5 2.5 2.5 2.5 Young'smodulus [GPa] 76 75 76 76 Load at which a crack is >2000 1800 1375 >2000developed with a probability of 50% [g] Example Item 19 Composition (%by weight) SiO₂ 65.3 Al₂O₃ 10.6 Li₂O 0.0 Na₂O 13.9 K₂O 0.0 MgO 3.1 CaO5.8 SrO 0.0 BaO 1.0 TiO₂ 0.0 ZrO₂ 0.3 R₂O 13.9 RO 9.9 Meltingtemperature (log η = 2) [° C.] 1561 Working temperature (log η = 4) [°C.] 1097 Devitrification temperature [° C.] 1106 Working temperature [°C.] − −9 Devitrification temperature [° C.] Glass Transition Point [°C.] 583 Coefficient of linear 86 expansion α (50 to 350° C.) [×10⁻⁷/°C.] Coefficient of linear 74 expansion α (−50 to 70° C.) [×10⁻⁷/° C.]Specific gravity [g/cm³] 2.6 Young's modulus [GPa] 75 Load at which acrack is developed with >2000 a probability of 50% [g](Preparation of Glass Substrates for Magnetic Recording Media)

First, using silica, alumina, lithium carbonate, sodium carbonate,potassium carbonate, basic magnesium carbonate, calcium carbonate,strontium carbonate, barium carbonate, titanium oxide and zirconiumoxide which are common glass raw materials, batches were prepared so asto result in compositions shown in Tables 1 and 2. Each of the batchesthus prepared was heated in an electric furnace by keeping each batch ina platinum crucible at 1550° C. for 4 hours to obtain a molten glass.The resultant molten glass was poured on an iron sheet outside thefurnace and cooled to form a glass block. After these glasses were keptat 650° C. for 30 minutes in the electric furnace, a power supply of thefurnace was turned off to allow them to cool slowly to room temperature,thereby obtaining sample glasses.

Each sample glass was processed into a columned form having a diameterof 5 mm and a length of 15 mm, and the coefficient of thermal expansionand the glass transition point thereof were measured with a differentialthermal dilatometer (Thermoflex TMA 8140, Rigaku).

Each sample glass was pulverized. Glass grains which had passed througha 2380-μm sieve and stayed on a 1,000-μm sieve were immersed in ethanol,and subjected to ultrasonic cleaning, followed by drying in athermostat. Twenty-five grams of the glass grains were placed in aplatinum boat having a width of 12 mm, a length of 200 mm and a depth of10 mm so as to give an approximately constant thickness, and kept in anelectric furnace having a temperature gradient of from 930 to 1180° C.Then, the grains were taken out of the furnace, and devitrificationgenerated in the inside of the glass was observed under a 40-poweroptical microscope. The maximum temperature at which devitrification wasobserved was taken as the devitrification temperature.

The sample glass was cut into a doughnut shape having an outer diameterof 68 mm and an inner diameter of 20 mm, and polished with aluminaabrasive grains. Both faces of the glass were further mirror-polished(surface roughness Ra: 2 nm or less; JIS B 0601-1994) with cerium oxideabrasive grains to form a 0.635-mm thick glass substrate (disk) formagnetic recording media. This disk was washed with a commercial alkalidetergent, and then immersed for 10 minutes in a molten salt ofpotassium nitrate heated at 440° C. to conduct a chemical strengtheningtreatment. In Comparative Example 1 in which the glass transition pointof the glass is low, the composition of salts and the temperature wereadjusted to the conditions disclosed in Japanese Patent No. 2,837,134,and the disk was immersed in a mixed molten salt of potassium nitrate(60%) and sodium nitrate (40%) for 10 minutes to conduct chemicalstrengthening. The disk was washed again with the commercial detergentto obtain a substrate for magnetic recording media. Using a diamondpenetrator (quadrangular pyramid-penetrator having an angle of 136degrees between opposite faces) of a micro Vickers hardness tester(MVK-G2, AKASHI CORPORATION), a load of 50 to 2000 g was applied to arecording surface of this substrate, and the pressure at which avertical crack is developed around the impression with a probability of50% was measured.

The glass composition was analyzed using wet chemical glass analysis incombination with atomic absorption spectro-photometry.

The specific gravity was measured by the Archimedes method, and theYoung's modulus was measured according to JIS R 1602 (the method fortesting the elastic modulus of fine ceramics).

In Examples of the invention, the coefficient of thermal expansion asmeasured in the range of from −50° C. to 70° C. was within the range of71×10⁻⁷/° C. to 82×10⁻⁷/° C., and 70×10⁻⁷/° C. or higher in all.

As shown in Tables 1 and 2, all glasses of Examples 1 to 19 of theinvention have a glass transition point of 560° C. or higher, and highin heat resistance compared to the glass of Comparative Example 1 whichhas a glass transition point of 491° C. Accordingly, the glasses of theinvention are excellent as members used at high temperatures orsubjected to high-temperature processes.

The glasses of Examples 1 to 19 of the invention have a pressure of 800g or more at which a crack is developed with a probability of 50%, afterthe chemical strengthening treatment, and have a high value compared tothe glass of Comparative Example 2 which is disclosed as a highheat-resistant glass. This reveals that the glasses having thecompositions of the invention have both the high heat resistance and thehigh degree of chemical strengthening.

In contrast, the glass of Comparative Example 1 has a glass transitionpoint as high as 615° C. and high heat resistance. However, the pressureat which a crack is developed with a probability of 50% is as low as 100g. Accordingly, the glass does not have both the high heat resistanceand the high degree of chemical strengthening (crack resistance).Further, the glass of Comparative Example 2 has a high degree of theload exceeding 2000 g, at which a crack is developed with a probabilityof 50%, but a glass transition point as low as 491° C. Similarly toComparative Example 1, therefore, the glass does not have both the highheat resistance and the high degree of chemical strengthening.

In Examples 8 to 16 and 19, (a value of the working temperature)—(avalue of the devitrification temperature) is −17° C. or lower. This isuseful in that the sheet glasses can be formed by the float processwithout occurrence of devitrification.

Preparation of Magnetic Recording Media

Then, using the glass substrates for magnetic recording media having thecompositions of Example 13, Comparative Examples 1 and 2, respectively,magnetic recording media were prepared in the following manner. Eachsample glass was cut into a doughnut shape having an outer diameter of68 mm and an inner diameter of 20 mm, and mirror-polished (surfaceroughness Ra: 2 nm or less; JIS B 0601-1994) by polishing of edges ofinner and outer peripheries, polishing of both faces (faces acting asrecording faces) with alumina abrasive grains, and polishing usingcerium oxide abrasive grains, thereby forming a 0.635-mm thick glasssubstrate.

These glass substrates were each washed with a commercial alkalidetergent. Then, the glass substrates of Example 13 and ComparativeExample 2 were each immersed for 4 hours in a molten salt of potassiumnitrate heated at 440° C. to conduct chemical strengthening, and furtherwashed again with the commercial alkali detergent. The glass substrateof Comparative Example 1 was washed with the commercial alkalidetergent, thereafter immersed in a mixed molten salt of potassiumnitrate (60 wt %) and sodium nitrate (40 wt %) for 4 hours to conductchemical strengthening, and washed again with the commercial alkalidetergent. The resultant glass substrates were each heated at 400° C.,and a Cr film, a Co—Cr—Ta alloy film, and a carbon film weresuccessively formed thereon as an undercoat layer, a recording layer anda protective layer, respectively, by sputtering. Further, the protectivelayer was coated with a fluorocarbon-based lubricating oil to prepareeach magnetic recording medium.

The resultant magnetic recording media were each subjected to arotational driving test using a test device based on a closed-typemagnetic recording device (HDD). In the rotational driving test, eachmagnetic recording medium was fitted and fixed on a stainless steelrotational shaft having a radius somewhat smaller than that of an innerperiphery of the magnetic recording medium, and driven for rotation at416.7 rounds per second (25,000 rpm). As a result, for the magneticrecording medium of Comparative Example 1, damage occurred duringrotation, which was conceivably caused by the insufficient degree ofchemical strengthening. However, for the magnetic recording media ofExample 13 and Comparative Example 2, such damage did not occur.

Then, the magnetic recording media were each subjected to a fixed-pointfloatation test and a continuous seek test. The fixed-point floatationtest was conducted under a reduced pressure of 26.7 kPa for 24 hours,and the presence or absence of head crushing was examined under anoptical microscope. The continuous seek test was conducted at a flyingheight of 15 nm at a rotation speed of 166.7 rounds per second (10,000rpm) for 1,000 hours, and the presence or absence of head crushing wasexamined under an optical microscope. For the magnetic recording mediausing the glass substrates obtained from the sample glasses of Example13 and Comparative Example 1, no head crushing error occurred. Incontrast, for the magnetic recording medium using the glass substrateobtained from the sample glass of Comparative Example 2, a head crushingerror caused by collision of the recording head with a recording faceoccurred frequently. The reasons why such difference was made are notclearly revealed. However, the reasons for this is conceivably that thehigh heat resistance of the glass causes no occurrence of warping anddeformation due to softening even when the glass is heated at hightemperatures in the formation process of the magnetic recording film,that minute strains formed on a surface of the glass by polishing arenot thermally relaxed, and that the alkali components (R₂O) in the glassare not deposited on the surface to form no minute projections.

Further, when the recording head flies over the magnetic recording face,slightly spaced therefrom, or runs while instantaneously touchingthereon, the presence of the minute projections on the surface of theglass generates more frictional heat. This heat is unfavorable becauseit causes thermal noises.

According to the glass article having the glass composition of theinvention, the composition of the glass is defined within the specifiedrange, so that the crack developing probability is reduced byapplication of the chemical strengthening treatment. Accordingly,mechanical strength having high reliability can be given. Further, evenwhen the glass is subjected to a high-temperature heat treatment, theglass is not deformed. Furthermore, no projections caused by alkalielution are formed on the surface thereof.

According to the glass substrate for magnetic recording media obtainedby processing the sheet glass having the glass composition of theinvention into a disk, even when the glass substrate is subjected tohigh-temperature heating in forming the magnetic recording layer on thesurface thereof, the heat resistance is high, thereby deformation of theglass is not generated, and also projections attributable to alkalielution on a glass surface are not formed on the surface of thesubstrate to allow the surface to be kept mirror-finished and smooth.

In addition, according to the glass substrate having the glasscomposition of the invention, even when the glass substrate is attachedto a metal rotational axis of a magnetic recording device and driven forrotation at high speed, occurrence of defects such as crack breakagecaused by changes in dimension by heat generated or vibration can beprevented, because the coefficient of thermal expansion of the glasssubstrate approximates to that of the metal, particularly stainlesssteel.

According to the raw sheet glass for obtaining the glass substrate formagnetic recording media of the invention, the glass composition iswithin the specified range, and the working temperature and thedevitrification temperature are selected so as to satisfy the specifiedrelationship, whereby the glass molten in the glass-melting furnace canbe directly introduced onto the molten tin bath, and formed into a sheetshape.

The entire disclosure of each and every foreign patent application fromwhich the benefit of foreign priority has been claimed in the presentapplication is incorporated herein by reference, as if fully set forth.

1. A chemically strengthened glass article obtained by immersing a glassarticle comprising a glass composition in a molten salt containingmonovalent cations having an ionic radius larger than that of Na ions toconduct an ion-exchange treatment between the Na ions and the monovalentcations, wherein the glass composition essentially comprises: 60 to 70wt % SiO₂; 8 to 15 wt % Al₂O₃; 0 to 1 wt % Li₂O; 8 to 16 wt % Na₂O; 0 to3.5 wt % K₂O 2 to 5 wt % MgO; 3 to 7.5 wt % CaO; 0 to 4.5 wt % SrO; and0 to 1 wt % ZrO₂, wherein the sum of Li₂O, Na₂O and K₂O is from 11 to 18wt %, and the sum of MgO, CaO and SrO is from 7 to 14 wt %.
 2. Thechemically strengthened glass article according to claim 1, which has aglass transition point of 560° C. or higher.
 3. The chemicallystrengthened glass article according to claim 1, which has a thermalexpansion coefficient of 70×10⁻⁷/° C. or higher as measured in the rangeof from −50° C. to 70° C., and a thermal expansion coefficient of80×10⁻⁷/° C. or higher as measured in the range of from 50° C. to 350°C.
 4. A chemically strengthened glass substrate for magnetic recordingmedia, which is obtained by immersing a glass substrate for magneticrecording media in a molten salt containing monovalent cations having anionic radius larger than that of Na ions to conduct an ion-exchangetreatment between the Na ions and the monovalent cations, wherein theglass substrate for magnetic recording media essentially comprises: 60to 70 wt % SiO₂; 8 to 15 wt % Al₂O₃; 0 to 1 wt % Li₂O; 8 to 16 wt %Na₂O; 0 to 3.5 wt % K₂O; 2 to 5 wt % MgO; 3 to 7.5 wt % CaO; 0 to 4.5 wt% SrO; and 0 to 1 wt % ZrO₂, wherein the sum of Li₂O, Na₂O and K₂O isfrom 11 to 18 wt %, and the sum of MgO, CaO and SrO is from 7 to 14 wt%.
 5. The chemically strengthened glass substrate for magnetic recordingmedia according to claim 4, which has a glass transition point of 560°C. or higher.
 6. The chemically strengthened glass substrate formagnetic recording media according to claim 4, which has a thermalexpansion coefficient of 70×10⁻⁷/° C. or higher as measured in the rangeof from −50° C. to 70° C., and a thermal expansion coefficient of80×10⁻⁷/° C. or higher as measured in the range of from 50° C. to 350°C.
 7. The chemically strengthened glass substrate for magnetic recordingmedia according to claim 4, wherein the load at which a crack isdeveloped with a probability of 50% by depressing a diamond penetratorof a micro Vickers hardness tester toward the substrate, is 800 g ormore.
 8. A method for producing a chemically strengthened sheet glass bya float process and subsequent chemical strengthening, which comprises:blending raw materials for a glass so as to give a molten glass havingthe following composition; and introducing the molten glass obtained bymelting the raw materials onto a bath of tin to form in a sheet shape:60 to 70 wt % SiO₂; 8 to 15 wt % Al₂O₃; 0 to 1 wt % Li₂O; 8 to 16 wt %Na₂O; 0 to 3.5 wt % K₂O; 2 to 5 wt % MgO; 3 to 7.5 wt % CaO; 0 to 4.5 wt% SrO; and 0 to 1 wt % ZrO₂, wherein the sum of Li₂O, Na₂O and K₂O isfrom 11 to 18 wt %, and the sum of MgO, CaO and SrO is from 7 to 14 wt%; and immersing the thus-formed glass sheet or portion thereof in amolten salt containing monovalent cations having an ionic radius largerthan that of Na ions to conduct an ion-exchange treatment between the Naions and the monovalent cations.
 9. The method according to claim 8,wherein the sheet glass has a glass transition point of 560° C. orhigher.
 10. The method according to claim 8, wherein the sheet glass hasa thermal expansion coefficient of 70×10⁻⁷/° C. or higher as measured inthe range of from −50° C. to 70° C., and a thermal expansion coefficientof 80×10−7/° C. or higher as measured in the range of from 50° C. to350° C.